1. Field of the Invention
The present invention relates to propylene homopolymers, preferably produced using a supercritical polymerization process.
2. Description of the Related Art
Since the mid-1980s metallocene catalysts have been used in high-pressure reactors-mainly for producing ethylene-backbone polymers including ethylene copolymers with monomers of one or more of propylene, butene, and hexene, along with other specialty monomers such as 4-methyl-1,5-hexadiene. For example U.S. Pat. No. 5,756,608, granted to Langhausen et al., reports a process for polymerizing C2 to C10 1-alkenes using bridged metallocene catalysts. Polypropylene production in high pressure conditions has, however, been seen as impractical and unworkable at temperatures much above the propylene critical point. A process to produce commercially useful polypropylene in a high pressure system would provide advantages, such as increased reactivity, or increased catalyst productivity, or higher throughput, or shorter residence times, etc. Likewise new polypropylene polymers are also in constant need for the preparation of new and improved products. Thus there is a need in the art to develop new processes capable of greater efficiency and manufacture of new polypropylene polymers.
U.S. Pat. No. 6,084,041, granted to Andtsjö et al., discloses supercritical propylene polymerization under relatively mild conditions (90-100° C. and less than 6.89 MPa pressure) using supported Ziegler-Natta and metallocene catalysts. This patent does not relate to propylene copolymerization at temperatures or pressures much higher than described above. It also does not specifically disclose bulk propylene polymerization using soluble, unsupported metallocene catalysts.
U.S. Pat. No. 5,969,062 granted to Mole et al., describes a process for preparing ethylene copolymers with α-olefins in which polymerization is carried out at a pressure between 100-350 MPa and at a temperature from 200-280° C. The catalyst is based on a tetramethylcyclopentadienyl titanium complex.
U.S. Pat. No. 5,408,017 describes an olefin polymerization catalyst for use at polymerization temperatures of 140° C.-160° C., or greater. Mainly, temperatures exceeding the melting point temperature and approaching the polymer decomposition temperature are said to yield high productivity.
WO 93/11171 discloses a polyolefin production process that comprises continuously feeding olefin monomer and a metallocene catalyst system into a reactor. The monomer is continuously polymerized to provide a monomer-polymer mixture. Reaction conditions keep this mixture at a pressure below the system's cloud-point pressure. These conditions create a polymer-rich and a monomer-rich phase and maintain the mixture's temperature above the polymer's melting point.
U.S. Pat. No. 6,355,741 discloses a process for producing polyolefins having a bimodal molecular weight distribution. The process comprises producing a first polyolefin fraction in a first loop reactor. The process couples this first loop reactor to a second loop reactor that prepares a second polyolefin fraction. At least one of the loops uses supercritical conditions.
WO 92/14766 describes a process comprising the steps of (a) continuously feeding olefinic monomer and a catalyst system, with a metallocene component and a cocatalyst component, to the reactor; (b) continuously polymerizing that monomer in a polymerization zone reactor under elevated pressure; (c) continuously removing the polymer/monomer mixture from the reactor; (d) continuously separating monomer from molten polymer; (e) reducing pressure to form a monomer-rich and a polymer-rich phase; and (f) separating monomer from the reactor.
U.S. Pat. No. 5,326,835 describes bimodal polyethylene production. This invention's first reactor stage is a loop reactor in which polymerization occurs in an inert, low-boiling hydrocarbon. After the loop reactor, the reaction medium transits into a gas-phase reactor where gas-phase ethylene polymerization occurs. The polymer produced appears to have a bimodal molecular weight distribution.
CA 2,118,711 (equivalent to DE 4,130,299) describes propylene polymerization at 149° C. and 1510 bar using (CH3)2C(fluorenyl)(cyclopentadienyl)zirconium dichloride complex, methylalumoxane and trimethylaluminum. Catalyst activity is reported to be 8380 gPP/g Ic′ h. The Mw is reported to be 2,000. CA 2,118,711 also describes propylene polymerization with ethylene at 190° C. and 1508 bar using (CH3)2C(fluorenyl)(cyclopentadienyl)zirconium dichloride complex, methylalumoxane and trimethylaluminum. Catalyst activity is reported to be 24,358 g Polymer/g Ic′ hr. The Mw is reported to be 10,000.
U.S. Pat. No. 7,279,536 discloses polypropylene polymers made at pressures around 200,000 kPa and temperatures from 109 to 175° C. using dimethyl rac-dimethylsilyl-bis-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-methyl-1H-benz(f)indene)hafnium dimethyl or dimethylsilyl bis(2-methyl-4-phenylindenyl)zirconium dichloride and methylalumoxane.
Other references include “Olefin Polymerization Using Highly Congested ansa-Metallocenes under High Pressure: Formation of Superhigh Molecular Weight Polyolefins,” Suzuki, et al., Macromolecules, 2000, 33, 754-759; EP 1 123 226; WO 00 12572; WO 00 37514; EP 1 195 391; U.S. Pat. No. 6,355,741; and “Ethylene Bis(Indenyl) Zirconocenes . . . ,” Schaverien, C.J. et al., Organometallics, ACS, Columbus Ohio, vol. 20, no. 16, August 2001, pg 3436-3452; WO 96/34023; WO 97/11098; U.S. Pat. Nos. 5,084,534; 2,852,501; WO 93/05082; EP 129 368 B1; WO 97/45434; JP 96-208535; U.S. Pat. No. 5,096,867; WO 96/12744; U.S. Pat. Nos. 5,408,017; 5,084,534; 6,225,432; WO 02/090399; EP 1 195 391; WO 02/50145; US 2002 013440; US 2004/0122191 A1; WO 2004/026921; WO 01/46273; EP 1 008 607; JP-1998-110003A; U.S. Pat. No. 6,562,914; and JP-1998-341202B2.
An abstract obtained from the Borealis website that states, “mPP produced in bulk conditions (100% propylene), especially at elevated temperature and under supercritical conditions, shows rheological behaviour indicative for small amounts of LCB in the polymer. This is a feature that can be utilized to produce mPP with enhanced melt strength under industrially meaningful conditions. “Barbo Loefgren, E. Kokko, L. Huhtanen, M Lahelin, Petri Lehmus, Udo Stehling. “Metallocene-PP produced under supercritical conditions,” 1st Blue Sky Conference on Catalytic Olefin Polymerization, 17-20/6/2002, Sorrrento, Italy, 2002.